The invention relates to a marine pipeline installation system for laying a pipeline and/or installing a subsea riser, said system at least being adapted to carry out the rigid reel lay method, wherein the system comprises:                one or more storage reels for storage of rigid pipeline to be installed,        a pipeline launch tower adapted for launching the pipeline in a firing line,        a pipeline guide supported at an elevated position by said tower for guiding said pipeline from a first trajectory extending between the reel and the pipeline guide to the firing line along said tower,        the pipeline guide being supported by the tower via an associated guide support frame, the support frame being adapted to allow for displacement of the pipeline guide between a pipelaying position, wherein one end of the pipeline guide is positioned in the firing line, and a retracted, non-operable position.        
Such a system is known e.g. from WO2007/108673 from the same applicant. The feature that the support frame is pivotably connected to the tower allows to remove the pipeline guide from the firing line. This is in particular useful when large accessories have to be installed in the pipeline. When removing the pipeline guide from the firing line the firing line is cleared to allow accessories to enter the firing line. The clearance may e.g. be required when the accessories are to enter the firing line from above, e.g when being lowered by a crane. In WO2007/108673 the marine pipeline installation system also comprises tensioners, which are retractable from the firing line for the same reason: to clear the firing line, here to allow accessories to enter the firing line.
A disadvantage of this pivotable support frame is that when the guide is being retracted, the first trajectory is effectively shortened. As a result, the back tension of the pipeline on the reel is reduced, which is undesirable as this leads to uncoiling of the pipeline, and cause pipeline portions to be squeezed. Another disadvantage may be that pipeline that has already been straightened, is retracted and subsequently led through the straightener again. However, rigid pipes cannot be straightened twice, as this leads to pipe deterioration and may be detrimental to the pipeline's quality.